The present invention is generally related to systems for calibrating transducers, and more particularly to circuits for producing negative voltage pulses used to generate electrostatic calibration forces for transducers used in sensing the acceleration profile of a moving object, such as a motor vehicle.
The prior art teaches transducers, or sensors, having a rigid frame and a sensing mass cantilevered on a support beam which is displaced generally normal to the direction of beam extension upon acceleration of an accelerating force to the frame. A plurality of strain gages bonded to the beam or diffused into the surface thereof are connected to a Wheatstone bridge in a manner familiar to those skilled in the art, whereby an output proportional to the amount of such sensing mass displacement is obtained. The Wheatstone bridge is typically referenced to a small positive excitation voltage, such as 5 volts, to accommodate natural sensing mass offset not caused by an accelerating force. Unfortunately, the output from the strain gages is adversely affected by creep and hysteresis losses. Moreover, the resistance of the strain gages and, hence, the output of the Wheatstone bridge connected thereto vary greatly with temperature. The variation in bridge output due to temperature is further complicated, for example, where the sensing mass and its supporting beam are micromachined from silicon, as the relationship between the acceleration-induced deflection of the sensing mass and temperature is unknown. Still further, such sensors are typically manufactured at high temperatures and subsequently cooled, whereby a thermal prestress is generated therein which is released or otherwise manifests itself as the operating temperature of the sensor varies. As a result, the sensor must be recalibrated on a continuing basis.
In the past, calibration of the sensor was achieved by periodically generating a predetermined, positive value, electrostatic displacement voltage. The displacement voltage is typically placed on the sensor beam in order to induce a particular electrostatic displacement field on the sensing mass. The particular electrostatic displacement field simulates the deflection effect on the sensing mass equivalent to application of a predetermined acceleration force to the sensor frame. The instantaneous change in the bridge output is then calibrated in terms of the simulated, predetermined acceleration value. Such calibration ensures that the sensor output accurately reflects true acceleration profiles of the moving object.
Typical prior art sensors, which are capable of output calibration, generally have been provided with electrical input leads on the sensor bridge to accommodate the above described positive value, electrostatic displacement voltage. However, in particular situations, such electrical input leads are susceptible to errant electrostatic voltages, such as static electricity discharged from a technician handling the sensor, thereby causing uncontrolled displacement of the sensing mass. In order to prevent this type of uncontrolled sensing mass displacement and possible attendant damage to the sensor, circuitry has been connected to the electrical input leads which reduces or eliminates any sensitivity of the sensor bridge to errant electrostatic voltages placed on the electrical input leads.
However, such circuitry is problematic for achieving sensor output calibration because the circuitry is typically realized as a diode arrangement which prevents any positive value electrostatic displacement voltages greater than the reference excitation voltage from deflecting the sensing mass. Accordingly, negative value electrostatic displacement voltages must be used to calibrate the sensor output as described hereinabove. Prior art negative voltage calibration pulse generating circuits have not presented a satisfactory solution due to the drawbacks of requiring a dedicated negative voltage power supply, and/or providing unregulated negative voltage outputs which can compromise accuracy in calibration of the sensor, or potentially damage the sensor due to the generation of excessive voltage spikes.